1、主将从现（主句为一般将来时,从句为一般现在时）； eg:If it rains(从句) ,I will stay at home（主句）. =I will stay at home if it rains. 2、主句含有情态动词 （must、may、can etc.）从句为一般现在时； eg:If you drive too fast ,you may have an accident....